Automatic and manual transmissions are commonly used on automotive vehicles, such as cars, trucks and Off-Highway Vehicles. Both conventional automatic and manual transmissions are restricted to a select few gear ratios, that enable a range of vehicle speeds while keeping the vehicle's internal combustion engine (ICE) operating within its limited operable engine speed range. Within the usable range of engine speeds for an ICE, there are optimal speeds for efficiency and power generation. Due to the discreet gear ratios of conventional automatic and manual transmissions, operating ICE vehicles at these optimal engine speeds is restricted to discreet vehicle speeds. Those transmissions are becoming more and more complicated since the engine speed has to be more precisely controlled to limit the fuel consumption and the emissions of cars. This finer control of the engine speed in usual transmissions can only be done by adding more gears (and corresponding discrete gear ratios) While adding additional gears to conventional transmissions can help the user operate the vehicle at optimal rpm ranges for a greater corresponding range of vehicle speeds, doing so adds significant cost and complexity to the transmission. Continuously variable transmissions (CVT) on the other hand can steplessly operate at an infinite number of gear rations between low gear ratio and a high gear ratio. CVTs are available in many types: belts with variable pulleys, toroidal, and conical to name a few. This ability to operate at a continuous range of gear ratios allows an automotive vehicle to operate at a constant ICE engine speed over a broad range of vehicle speeds. The main advantage of a CVT is that it enables the engine to run at its most efficient rotation speed by changing steplessly the transmission ratio as a function of the vehicle speed. Moreover, the CVT can also shift to a ratio providing more power if higher acceleration is needed. A CVT can change the ratio from the minimum to the maximum ratio without any interruption of power, unlike conventional transmissions which cause an interruption of power during ratio shifts. Furthermore, such capabilities allow for the optimization of the ICE design for narrow but more efficient power bands, allowing greater useable power from smaller displacement more economical engines. A specific use of CVTs is the Infinite Variable Transmission or IVT. Whereas the CVT is limited at positive speed ratios, the IVT configuration can perform a neutral gear and even reverse ratios steplessly. A CVT can also be used as an IVT in some driveline configurations.
A typical CVT design example is the Fallbrook “NuVinci” Technology, which is a rolling traction drive system, transmitting forces between the input and output rolling surfaces through shearing a thin fluid film. NuVinci designs utilize a continuously variable planetary (CVP) variator, which steplessly operates through a range of speed ratios. The technology is called “Continuously Variable Planetary” (CVP) due to its analogous operation to a planetary gear system. The system consists of an input disc (ring) driven by the power source, an output disc (ring) driving the CVP output and a set of balls rotating on its own axle and is fitted between these two discs and a central sun.
The torque from the input power source is transferred between input ring, balls and output ring using a thin layer of traction fluid (elasto-hydrodynamic lubrication, or EHL). The discs are clamped onto the balls tightly to achieve the clamping force required to transmit the torque.
The relative speed of the output ring is controlled by tilting the angle of the ball axles relative to the transmission axis. By tilting the ball axles the CVP can operate steplessly within a range of speed ratios. Typically the speed ration range spans underdrive to overdrive ratios.
One challenge in using a CVT, such as those having a CVP, is the management of high torques supplied to the CVT. Over-torque situations wherein torque flowing through the CVT surpasses the design limit of the variator or other components of the transmission may lead to catastrophic failure, damage, or decreased operating life of the variator or other transmission components. Currently there exists no cost-effective solution that can detect high torque high and adequately control the variator in such cases. Thus, the variator will not be protected and oversizing might be needed to ensure the sufficient life of the designs. Thus there exists a need for a system that can detect high torques being supplied to the variator and that can adequately control the variator or other aspects of the vehicle driveline to prevent damage to and preserve the operating life of the transmission, especially the variator.